I've read this in Saint Thomas Aquinas' sermons on the Apostles' Creed (emphasis mine):
“Confirmation.”—The second Sacrament is Confirmation. Just as they who are physically born need certain powers to act, so those who are reborn spiritually must have the strength of the Holy Spirit which is imparted to them in this Sacrament. In order that they might become strong, the Apostles received the Holy Spirit after the Ascension of Christ: “Stay you in the city till you be endowed with power from on high” [Lk 24:49]. This power is given in the Sacrament of Confirmation. They, therefore, who have the care of children should be very careful to see that they be confirmed, because great grace is conferred in Confirmation. He who is confirmed will, when he dies, enjoy greater glory than one not confirmed, because greater grace will be his.
Thomas speaks about "children" to be confirmed. The common practice today is to confirm young adults, to mark their maturity and mature acceptance of their faith. Thomas seems to advocate the other concept of the sacrament of confirmation: this sacrament should open the young boy or girl to the help of Holy Spirit to become a true adult Christian.
When should young Christian be confirmed according to st. Thomas? Or in other words: what were the common ages when people were confirmed in 13th century? Of these, it's easy to pick those that can be seen as "children".